1. Field of the Invention
The invention relates to an apparatus for detecting an inter-cylinder variation abnormality in the air/fuel ratio, and a detection method for the apparatus. Particularly, the invention relates to an apparatus able to detect that in a multi-cylinder internal combustion engine, the air/fuel ratio is varying among the cylinders to a relatively great extent, and a detection method for the apparatus.
2. Description of the Related Art
Generally, with regard to an internal combustion engine equipped with an exhaust control system that uses catalysts, in order to achieve high-efficient removal of pollutants from exhaust gas, it is essential to control the mixing rate between air and fuel in a mixture that is burned in the internal combustion engine, that is, control the air/fuel ratio. In order to perform the control of the air/fuel ratio, an air/fuel ratio sensor is provided in an exhaust passageway of the internal combustion engine, and a feedback control is performed so that the air/fuel ratio detected by the sensor becomes equal to a predetermined target air/fuel ratio.
Usually, in a multicylinder internal combustion engine, the air/fuel ratio control is performed by using the same control amount for all the cylinders; therefore, despite execution of the air/fuel ratio control, the actual air/fuel ratio sometimes varies among the cylinders. In such a case, if the variation in the air/fuel ratio is of a small degree, the variation in the air/fuel ratio can be absorbed by the feedback control of the air/fuel ratio and pollutants in exhaust gas can be removed by the catalysts. Thus, small degrees of the variation in the air/fuel ratio do not affect the exhaust emission, and do not cause any particular problem.
However, if the air/fuel ratio greatly varies among the cylinders due to, for example, failure of the fuel injection systems of one or more cylinders, etc., the exhaust emission deteriorates, and problems arise. It is desirable that such a large variation in the air/fuel ratio that deteriorates the exhaust emission be detected as an abnormality. Particularly, in the case of the internal combustion engines for use in motor vehicles, in order to prevent a vehicle from traveling with deteriorated exhaust emission, detection of inter-cylinder air/fuel ratio variation abnormality in a vehicle-mounted (on-bard) engine has been demanded, and is recently being made a legal requirement in some countries.
For example, an apparatus described in Japanese Patent Application Publication No. 2009-180171 (JP 2009-180171 A) detects abnormality in the variation in air/fuel ratio among the cylinders of an internal combustion engine on the basis of fluctuations of the air/fuel ratio of the engine. Furthermore, with regard to a plurality of fuel injection valves provided for each of the cylinders, the injection proportion among the plurality of fuel injection valves is altered, and it is discerned which of the fuel injection valves bears the cause of the variation abnormality on the basis of fluctuation of the air/fuel ratio between before and after the alteration in the injection proportion.
However, the construction described in Japanese Patent Application Publication No. 2009-180171 (JP 2009-180171 A) is able only to discern the presence or absence of the abnormality and which of the fuel injection valves has the abnormality, and is not able to specifically determine the degree of the abnormality.